Since the invention of printing technology, industry has endeavoured to devise techniques to combat forgery of printed documents. For instance, there are continuing efforts to prevent forgery of bank notes.
Recently, the advent of inexpensive, high-resolution scanning and printing equipment has improved the quality of the forged documents and made it increasingly difficult to detect forged printed documents. Furthermore, the popularity of printed documents as value-bearing instruments is increasing and, hence, printed documents are an appealing target for would-be forgers. For example, forged cheques and tickets often remain undetected.
It is, therefore, desirable to be able to authenticate printed documents. Further, it is desirable that the authentication methods are automated. Such automation will provide greater accuracy, consistency and speed.
One known class of methods for document authentication involves placing a mark that is difficult to replicate on the document. Examples of such marks are holographic emblems and embedded watermarks. Forgers require expensive equipment and a great deal of knowledge to be able to successfully replicate a document authenticated in such manner. However, such marks are expensive to produce and may not be machine-readable.
Another known class of known methods aims to prevent forgery by encoding machine-readable marks on the surface of the document. Upon detection of these machine-readable marks on a document, the copying machine is instructed to halt replication of the document. These marks are inexpensive to reproduce, since they may be printed with a conventional printer. However, a forger may degrade the machine-readable marks to a point where the human-readable document content is satisfactorily reproduced, yet the machine-readable marks are undetectable by the copying machine.
A further class of methods involves encoding a printed image in a barcode and printing the barcode over the printed image. The printed image may be a photograph of a person's head, such as those often used in drivers' licenses. This protects the photograph from tampering since any changes to the photograph would result in a mismatch between the barcode data and the altered photograph. However, these methods afford little security from forgery in which the photograph and barcode are replicated without alteration.
A further class of methods identifies characteristics of a region of the substrate of the document (which is typically paper) and stores centrally information about the identity of document and about the corresponding paper characteristic. When a document is to be authenticated, its paper is again characterised, and compared to the reference paper characteristic stored previously. These methods are very effective at detecting forged documents since each paper characteristic is unique. However, if a large number of documents are to be protected, a large set of paper characteristics will need to be stored centrally. The remote access to a centralized server with a database of security documents may also be a security problem.
Recent methods have been devised to overcome the need to centrally store the paper characteristics. These methods involve encoding the paper characteristics of a document in a barcode encoded somewhere on the document itself. With this technique, the document is considered to be self-verifying since all the information needed to authenticate the document is carried on the document. One disadvantage of these methods is that the region occupied by the barcode and the region of paper that is characterized, limit the amount of user-related data that can be printed on the document. Furthermore, the set of synchronization marks required for each of these regions are highly visible and are, therefore, susceptible to tampering. However, if synchronization marks are only used for the barcode, and the paper characteristic region is specified relative to the barcode, local warping and distortions may hamper accurate characterization of the paper region.
Thus, a need exists for more efficient methods of generating and authenticating security printed documents.